The invention relates generally to improvements in television receivers. It is particularly directed to a pulse width modulator for varying the width of horizontal rate pulses which are further processed to deflect the receiver's electron beam or beams.
Some television receivers include a pulse width modulator which receives horizontal rate pulses from a pulse generator and modifies the width of those pulses in response to certain other inputs. The width-modulated pulses may then be applied to a horizontal output transistor which drives the receiver's yoke to deflect the receiver's electron beam, and which also drives a flyback transformer from which the receiver's high voltage is derived. If the widths of the modulated pulses are properly controlled to control the on-time of the horizontal output transistor, the high voltage will be held constant. Consequently, the horizontal size of the picture can be held reasonably constant despite variations in line voltage.
The high voltage generating circuit discussed above is commonly referred to as a switched-mode power supply. Generally, all the receiver's circuits are energized, directly or indirectly, from this supply, including the generator which develops the pulses and the modulator which varies the width of the pulses. Hence, when the receiver is first turned on, it is necessary to temporarily provide the pulse generator and the modulator with power other than that normally provided by the switched-mode power supply (which is initially inoperative when the receiver is first turned on).
For this purpose, such receivers commonly include a start-up transformer which is energized by line voltage as soon as the receiver is turned on. The voltage developed by this transformer is used to temporarily power the pulse generator and modulator to start the switched-mode power supply. When the latter supply is operative, the start-up transformer is deactivated and the pulse generator and modulator are powered by the switched-mode power supply.
For reasons of economy, the start-up transformer is usually selected to be as small as possible. Consequently, its load regulation is poor. In addition, it usually develops a low output voltage under conditions in which the AC line voltage is lower than normal. Consequently, the pulses developed by the power generator are of an abnormally low amplitude. Because of these factors, special consideration must be given to the design of the modulator which receives the output of the pulse generator and which supplies width-modulated pulses to the horizontal output transistor.
Specifically, before the start-up transformer is deactivated, the modulator should be: (1) in a state which deactivates circuitry downstream of the modulator so as to reduce the load on the start-up transformer and ensure that the pulse generator is adequately powered to begin developing its output pulses; and (2) biased symmetrically, i.e., with no offset, so that its sensitivity to input pulses is high. This will ensure the production of output pulses even when the modulator receives pulses of low amplitude from the pulse generator.
A further consideration relates to the degree to which the modulator alters the widths of the horizontal rate pulses. If the modulator can output a pulse with a zero or a 100% duty cycle, horizontal scan and the high voltage which is usually derived from the horizontal deflection circuitry may be shut down. Such may occur, for example, when the line voltage powering the receiver undergoes a sudden decrease and the modulator responds by generating an essentially D.C. output.
Prior pulse width modulators have not satisfactorily responded to the foregoing considerations. In addition, conventional pulse width modulators, operating as they do in a substantially open loop mode, have not operated with the desired degree of consistency. The present invention provides a pulse width modulator which meets the requirements discussed above and operates to maintain a substantially constant picture size as line voltage changes and as the receiver's beam current changes.